This invention relates to battery operated converters for driving an electroluminescent (EL) lamp and, in particular, to a half-bridge converter having an alternating current (AC) ground.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by a converter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
The prior art discloses several types of inverters in which the energy stored in an inductor is supplied to an EL lamp as a small current at high voltage as the inductor is discharged through the lamp. The voltage on the lamp is pumped up by a series of pulses from the inverter. The direct current produced by inverter must be converted into an alternating current in order to power an EL lamp. U.S. Pat. No. 4,527,096 (Kindlmann) discloses a switching bridge for this purpose. U.S. Pat. No. 5,313,141 (Kimball) discloses an alternative design that produces an AC voltage directly.
It is also known in the art to drive an EL lamp with a push-pull output rather than a full bridge as in the Kindlmann patent; see U.S. Pat. No. 5,566,064 (Shoenwald et al.). Although the Shoenwald et al. patent refers to a bridge, the circuit is actually push-pull because the emitters of the switching transistors are grounded; that is, one terminal of what would be an AC diagonal of a bridge circuit is grounded. In an actual bridge circuit, one end of the DC diagonal is grounded.
Half-bridge inverters are known in the power supply art, e.g. U.S. Pat. Nos. 4,533,986 (Jones) and 5,060,130 (Steigerwald). These patents describe switching power supplies producing several watts of power, or more, and having direct current outputs.
Many applications for EL lamps, such as watches and cellular telephones, require small, inexpensive drivers. The Kimball design requires dielectric isolation to implement, a relatively expensive process. The full bridge requires four switching transistors, which makes the die relatively large and, therefore, expensive. The push-pull design described in the Shoenwald et al. patent has only two switching transistors but requires at least nine pins to implement. It is desired to minimize pin count, in addition to minimizing die size, the combination of which can significantly reduce the cost of a packaged device. The Shoenwald et al. design also requires more than one inductor, making the total cost of the circuit extremely high relative to other designs.
There are relatively simple circuits known in the art for driving an EL lamp and such circuits, if implemented as an integrated circuit, might have few pins. However, these circuits compromise operation, typically by producing a DC bias on the EL lamp, which greatly shortens the life of the lamp.
In view of the foregoing, it is therefore an object of the invention to provide a true half-bridge inverter for EL lamps.
Another object of the invention is to provide an inverter that can be implemented in a junction isolation process, among others.
A further object of the invention is to provide an inverter for an EL lamp that can be implemented as an integrated circuit with fewer pins than circuits in the prior art with the same functionality.
Another object of the invention is to provide an inverter for an EL lamp that can be implemented as an integrated circuit of small die size.
The foregoing objects are achieved in this invention in which an inverter for driving an EL lamp includes a half-bridge output.